Tensions between Russia and Ukraine escalated sharply on Friday as Moscow sent more than 130 trucks rolling across the border in what it said was a mission to deliver humanitarian aid. Ukraine called it a "direct invasion," and the U.S. and NATO condemned it as well.

In another ominous turn in the crisis, NATO said it has mounting evidence that Russian forces are operating inside Ukraine and launching artillery attacks from Ukrainian soil.

The trucks, part of a convoy of 260 vehicles, entered Ukraine without government permission after being held up at the border for a week amid fears that the mission was a Kremlin ploy to help the pro-Russian separatists in eastern Ukraine.

The Russian ambassador to the UN denied the claims.

"We discussed providing humanitarian aid. This is what Russia is doing in this particular case. The United States does not have a monopoly on humanism," said Ambassador Vitaly Churkin. 

"There was ample possibility for double checking on the convoy. Journalists were shown the cargo. We went into very detailed discussion with ICRC (International Committee of the Red Cross) professionals."

The White House National Security Council said the convoy comprised of "Russian military vehicles painted to look like civilian trucks."

"While a small number of these vehicles were inspected by Ukrainian customs officials, most of the vehicles have not been inspected by anyone but Russia," spokesperson Caitlin Hayden said in a statement released Friday. 

The statement was made shortly after the White House denounced the convoy's move into Ukraine as a flagrant violation of Ukraine's sovereignty.

"We are deeply concerned about this," said Ben Rhodes, deputy national security adviser to President Barack Obama.

Rhodes said the United States planned to discuss the situation with the UN Security Council on Friday. He said if the convoy is not removed, the Russians will face "additional costs," meaning sanctions that have been levied against the Russian economy.

"Russia must remove its vehicles and its personnel from the territory of Ukraine immediately," the Pentagon's press secretary, Navy Rear Adm. John Kirby told reporters earlier. "Failure to do so will result in additional costs and isolation."

In a closed-door emergency meeting at the UN, several countries rebuked Russia for "what many called an illegal and unilateral action by the Russian federation," British Ambassador Mark Lyall Grant, the council president, told reporters.

"It is an undeniable and blatant violation of Ukrainian sovereignty," Lyall Grant said. "It has nothing to do with humanitarianism. That humanitarian effort is being coordinated by the UN and if the Russia federation wanted to participate in that, it could have done so in a collective way rather than acting unilaterally."

Lyall Grant said "there was no unanimity of views" during the emergency consultations, which were held at the request of Lithuania.

Earlier, a spokesperson for UN chief Ban Ki-moon echoed U.S. concerns, saying the envoy's move has "the potential of exacerbating an already dangerous situation." 

Prime Minister Stephen Harper on Friday called the move "disgraceful," especially because it was an incursion "under the guise of humanitarian assistance."

Russian aid trucks might be partially filled

By late afternoon, trucks had reached the city of Luhansk, whose war-reduced population of a quarter-million people has suffered under intense fighting over the past several weeks between Ukrainian forces and the separatists.

Russia said the white-tarped vehicles were carrying food, water, generators and sleeping bags. 

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A truck driver from the Russian aid convoy stands in the Russian inspection zone inside a border control point with Ukraine in the Russian town of Donetsk. (Sergei Grits/Associated Press)

Some of the trucks were opened to reporters a few days ago, and at least some of those items could be seen. But Associated Press journalists following the convoy across rough country roads heard the trucks' contents rattling and sliding around Friday, suggesting many vehicles were only partially loaded.

The arrival of the trucks instantly raised the stakes in the crisis: An attack on the convoy could give Russia a pretext to intervene more deeply in the fighting. And the convoy's mere presence could block further battlefield advances by Ukrainian forces, which have reported substantial inroads against the rebels over the past week.

In sending in the convoy, Russia said it had lost patience with Ukraine's stalling tactics and claimed that soon "there will no longer be anyone left to help" in Luhansk, where weeks of heavy shelling have cut off power, water and phone service and made food scarce.

Moscow's decision to move unilaterally, without Red Cross involvement, raised questions about its intentions.

Suspicions were running high that the humanitarian operation may instead be aimed at halting Kyiv's momentum on the battlefield.

Ukrainian Prime Minister Arseniy Yatsenyuk declared that the trucks were half-empty and were not going to deliver aid but would instead be used to create a provocation. He said Russia would somehow attack the convoy itself, creating an international incident.

Red Cross refuses to escort trucks

Ukrainian security services chief Valentyn Nalyvaichenko called the convoy a "direct invasion" and said the half-empty trucks would be used to transport weapons to rebels and spirit away the bodies of Russian fighters killed in eastern Ukraine. He said the men operating the trucks were Russian military personnel trained to drive combat vehicles, tanks and artillery.

Nalyvaichenko insisted, however, that Ukraine would not shell the convoy. 

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Trucks of a Russian convoy carrying humanitarian aid for Ukraine drive before parking at a camp near Donetsk, Ukraine. (Alexander Demianchuk/Reuters)

NATO's secretary-general condemned Russia for sending in a "so-called humanitarian convoy." Anders Fogh Rasmussen said Russia committed "a blatant breach" of its international commitments and "a further violation of Ukraine's sovereignty."

NATO spokeswoman Oana Lungescu said that, since mid-August, the military alliance has seen multiple reports of direct involvement of Russian forces in Ukraine, along with transfers of tanks and other heavy weapons to the separatists, and "an alarming build-up of Russian ground and air forces in the vicinity of Ukraine."

"Russian artillery support — both cross-border and from within Ukraine — is being employed against the Ukrainian armed forces," she said. Previously, the West accused Russia of cross-border shelling.

The Red Cross, which had planned to escort the convoy to assuage fears that it was a cover for a Russian invasion, said it had not received enough security guarantees to do so, as shelling had continued overnight. Four troops were killed and 23 wounded in a 24-hour period in eastern Ukraine, the government reported Friday.

Number of trucks surpasses authorized amount

The government said it had authorized the entry of only 35 trucks. But the number of Russian vehicles seen passing through was clearly way beyond that. International monitors said that as of midday, 134 trucks, 12 support vehicles and one ambulance had crossed into Ukraine.

In announcing its decision to act, the Russian Foreign Ministry said: "There is increasingly a sense that the Ukrainian leaders are deliberately dragging out the delivery of the humanitarian load until there is a situation in which there will no longer be anyone left to help." 

It added: "We are warning against any attempts to thwart this purely humanitarian mission."

Rebel forces took advantage of Ukraine's promise not to shell the convoy to drive on the same country road as the trucks. Some 20 green military supply vehicles — flatbed trucks and fuel tankers — were seen travelling in the opposite direction, along with smaller rebel vehicles. 

The fighting in eastern Ukraine began in mid-April, a month after Russia annexed Ukraine's Crimean Peninsula. It has killed over 2,000 people and forced 340,000 to flee, according to the United Nations.

On Friday, Lithuanian Foreign Minister Linas Linkevicius said the country's honorary consul in Luhansk had been abducted and killed by "terrorists." There were no further details.

With files from Reuters and CBC News